1. Field of the Invention
The present invention relates to an image signal processing device for example for converting input image signals into binary signals.
2. Description of the Prior Art
In image processing apparatus such as facsimile, it is already known to handle image signals in the form of binary signals indicating white and black levels.
There are already proposed various binary digitizing circuits for image signals, as exemplified in FIG. 1.
In the conventional circuit shown in FIG. 1, image signals obtained from unrepresented image taking means such as a CCD linear sensor are supplied, in addition to a comparator circuit 1, to an envelope detector 2 in which the envelope of white level is detected by a half-wave rectifier with a suitable time constant.
The obtained envelope output signal is divided in a voltage-dividing circuit 3, and, after the addition of an offset voltage in a DC voltage adder circuit 4 in order to avoid noise in the black area, is supplied as a reference voltage to the comparator circuit 1 for binary digitizing of the aforementioned image signals.
In the above-described conventional circuit the envelope detector 2 has a function as a peak-hold circuit for the white level, and such circuit is known to be associated with the drawback that the quality of the image formed by such binary signals is delicately affected by the selection of the discharge time constant of said peak-hold circuit. Also in FIG. 1 it is assumed that the black level of the original document corresponds to the ground potential, and there will be required a circuit for clamping the black level to the ground potential. Also the binary digitizing of image signals representing white characters on a black background cannot be effected satisfactorily because of the simple addition of the offset voltage by the DC voltage adder circuit 4.
Also in a circuit in which the slicing level for binary digitizing is determined in response to the image signals, the presence of noises in the image signals may affect the slicing level, thus disturbing normal digitizing operation.